Method for harvesting rare earth barium copper oxide single crystals

ABSTRACT

A method of preparing high temperature superconductor single crystals. The method of preparation involves preparing precursor materials of a particular composition, heating the precursor material to achieve a peritectic mixture of peritectic liquid and crystals of the high temperature superconductor, cooling the peritectic mixture to quench directly the mixture on a porous, wettable inert substrate to wick off the peritectic liquid, leaving single crystals of the high temperature superconductor on the porous substrate. Alternatively, the peritectic mixture can be cooled to a solid mass and reheated on a porous, inert substrate to melt the matrix of peritectic fluid while leaving the crystals melted, allowing the wicking away of the peritectic liquid.

This invention was made with Government support under Contract No.W-31-109-ENG-38 awarded by the Department of Energy, and the U.S.Government has certain rights in this invention.

The present invention is concerned generally with a method of producingsingle crystals of high temperature superconductor. More particularly,the invention is concerned with a method of harvesting high temperaturesuperconductor crystals from a peritectic flux melt by wicking away, ordesorbtion, of the melt and collecting the solid single crystals on aninert substrate.

While great promise exists for the utilization of high temperaturesuperconductors, many important commercial applications requirepreparation of large single crystals of the high temperaturesuperconductor (HTSC). The most promising technique for preparation ofsuch large single crystals involves production of HTSC seed crystals tohelp grow HTSC masses containing large, aligned HTSC grains. Such seedcrystals can be, for example, a (RE)Ba₂ Cu₃ O_(x) where (RE) is Sm or Ndin place of Y. Other prior art methods involve difficult techniques suchas etching away the matrix surrounding HTSC seed crystals, leaving theseed crystals behind.

It is therefore an object of the invention to provide an improved methodof manufacture of HTSC seed crystals.

It is another object of the invention to provide a novel method ofcollecting HTSC single crystals by forming a liquid/solid mixture andremoving the liquid, leaving solid seed crystals behind.

It is a further object of the invention to provide an improved method ofcollecting HTSC single crystals from a liquid by absorbtion of theunwanted liquid by an inert, porous substrate.

It is also an object of the invention to provide a novel method ofproducing HTSC single crystals of Nd (or Sm) Ba₂ Cu₃ O_(x) by heating asolid mass to a temperature sufficient to form a flux melt removable byabsorbtion.

It is an additional object of the invention to provide an improvedmethod of collecting HTSC single crystals by forming a mixture of singlecrystals and equilibrium liquid phase over an inert, porous HTSCsubstrate which absorbs the equilibrium liquid phase.

It is yet another object of the invention to provide a novel method ofcollecting HTSC seed crystals by removal of a liquid phase surroundingthe seed crystals by capillary desorbtion.

Other objects and advantages will become apparent from the detaileddescription and drawings described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 sets forth a flow diagram illustrating a preferred method of theinvention; and

FIG. 2 is a pseudo ternary phase diagram of Nd₂ O₃, BaO and CuO showingtwo particular preferred compositions of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A method of the invention is described generally by reference to theflow diagram of FIG. 1. Seed crystals of a high temperaturesuperconductor (HTSC, hereinafter) can be produced by the method of theinvention for any one of the HTSC systems which exhibits a peritecticphase diagram. Given such a system, one proceeds by procuring thestarting chemicals, elemental or compounded, and weighing out theappropriate amounts to achieve the desired composition. Alternatively,the precursor material can be an already compounded material adequate toachieve the desired result upon heating to attain the peritecticequilibrium mixture described hereinafter. Ideally the precursormaterial is such that one achieves optimum growth conditions for thedesired HTSC seed crystals. It is also preferable to minimize the mountof liquid used to suspend the crystals for growth, but not such a smallamount as to cause the crystals to form together, which would defeat theobjective of obtaining isolated single crystals. The end morphology ofcrystals can also be modified in a conventional manner by changing thecomposition. As is known in the crystal growing art, the kinetics ofgrowth vary within such a peritectic phase region; and as explained forExample II, one can, for example, adjust the crystal growth conditionsto change from a flat-sided crystal to cubic-shaped crystal morphology,depending primarily on the compositional location in the peritecticphase region. Such parameters as composition, temperature and quenchingconditions can thus be readily adjusted by one of skill in the art toobtain the desired particular crystal morphology, as well as the numberand size of seed crystals obtained.

In the method of the invention, therefore, the composition can be suchas to generally place the material within a peritectic phase regionwhich includes the HTSC composition plus a liquid phase. Virtually allthe HTSC phase diagrams have such a phase region, enabling utilizationof the method of this invention in all such HTSC systems.

Referring again to FIG. 1 the selected precursor material amounts areground up in preparation for compounding. A typical method use ball millgrinding of the materials in isopropanol (or other such inert liquid)for 4-12 hours. The resulting ground powder materials are dried, such asby a conventional furnace or hot plate; and then the powder materialsare pressed into pellets. These pressed pellets are precalcined in theconventional manner in preparation for heating to the desired positionof the phase diagram in the peritectic phase region.

A substrate must also be prepared for the purpose of receiving theperitectic liquid plus HTSC crystal mixture. This substrate has theproperties of being inert to the HTSC peritectic liquid to be wickedaway, and also has a melting point above the melting temperature of theperitectic liquid phase. Such substrates are prepared to be wettable bythe liquid and also to exhibit porosity, thereby enabling theabsorbtion, or wicking away by the substrate, of the peritectic liquidfrom the mixture containing the HTSC seed crystals. For example,substrates can be prepared of a porous pellet, such as an HTSC or othernonsuperconducting phase, with melting point higher than the desiredHTSC single crystals. Typically, such solid materials as Y₂ BaCuO_(x)(211 YBaCu Oxide) can be used for the 123 Nd (or Sm) BaCu oxide systems.For the Y₂ BaCuO_(x) system, the pellets are sintered at about 30% belowthe melting point for 8-48 hours.

In the next step the precalcined pellets of the precursor material areplaced in an inert crucible, such as Al₂ O₃, a noble metal or othernonreactive material. This crucible and precursor material are heated toa temperature for a time adequate to insure formation substantially ofthe equilibrium phases in the peritectic phase diagram. The heatingprotocol (and gas atmosphere conditions) can follow a wide variety ofconventional thermodynamic paths but should result in achieving theprescribed equilibrium phase state needed in order to wick away theperitectic liquid flux, leaving behind the HTSC seed crystals.

Upon achieving the desire equilibrium phase mixture, the seed crystalsare allowed to grow to achieve the desired size and number bycontrolling the crystal growth conditions in a well-known, conventionalmanner. The contents of the crucible then can be processed in one of twopreferred ways: (1) directly depositing the HTSC seed crystal/peritecticliquid mixture onto the above described porous substrate or (2)quenching the peritectic liquid/seed crystal mixture to form a solidmass, depositing the solid mass onto the porous substrate, and heatingthe mass sufficiently to melt only the solid matrix originally formedfrom the peritectic liquid, and wicking off that liquid. After removingthe unwanted peritectic liquid (flux), one can physically remove theremaining HTSC seed crystals left sitting on the porous substrate.

The following nonlimiting Examples illustrate various illustrations ofthe method of the invention.

Example I

A method of harvesting Nd_(1+x) Ba_(2-x) Cu₃ O_(y) crystals includes thefollowing steps: (1) reagant grade Nd₂ O₃, CuO and Ba CO₃ are weighed inthe composition ratio of 1:10:4 (mole percent), (2) these startingprecursor materials are ball milled in a polyethylene bottle with azirconia milling media in ethanol or isopropanol for 4-12 hours toachieve a homogeneous mixture of starting materials, (3) the powderedstarting materials are dried on a hot plate under a N₂ atmosphere toavoid reactions of the starting powder with ambient atmosphere, (4) thedried powders are pressed into a pellet to reduce contact of the pelletwith the alumina crucible to be used to melt the precursor materials;typical pellet diameters are 20 mm diameter, have a mass of 20 g and arepelletized using a pressure of 5,000-10,000 Pa, (5) the pellets arepresintered at 940° C. for a least 24 hours, (6) are then heated toabout 1050° C.-1100° C. and held for about 48 hours, (7) the heatedmixture is cooled slowly (about 1° to 10° C. an hour) to about 1000° C.to promote crystal growth in the peritectic liquid (flux), (8) thecrucible containing the specimen is quenched onto a copper block to formsolid mass, stopping crystal growth, (9) place solid mass present in thecrucible onto a porous Y₂ BaCuO_(x) substrate, (10) heat the solid massto about 1050° C., hold for 2-8 hours allowing peritectic liquid (flux)to be absorbed, or wicked, away into the porous structure by grivitationand capillary forces, (11) the remaining crystals are cooled to roomtemperature and (12) the seed crystals are removed from the poroussubstrate.

Example II

In the Nd₂ O₃ /CuO/BaO system (See FIG. 2) single crystals were preparedof different morphologies by adjusting composition within the peritecticphase region. 1/2(Nd₂ O₃),BaO and CuO were mixed in the following tworatios: 1:4:12.5 and 1:2:6.25. Each of these compositions were heated toa temperature of 950° C. and held for 48 hours. At the end of that timethe peritectic mixture was processed in the same manner as Example I tocollect the crystals. In the case of the ratio being 1:4:12.5, thecrystals had a flat morphology. In the case of the ratio being 1:2:6.25,the morphology was cube shaped for the crystals.

Further advantages and features of the invention will be appreciated byreference to the claims set forth hereinafter. While preferredembodiments have been described, it will be clear to those of ordinaryskill in the art that changes and modifications can be made withoutdeparting from the spirit and scope of the invention in its fullestaspects.

What is claimed is:
 1. A method of preparing high temperature superconductor single crystals, comprising the steps of:preparing a precursor material having a selected composition; heating said precursor material in a crucible and achieving a peritectic equilibrium mixture of phases including a peritectic liquid and crystals of high temperature superconductor; cooling said peritectic equilibrium mixture in said crucible to form a solid mass in said crucible; removing said solid mass from said crucible; positioning a substrate having a porosity and being inert to said peritectic equilibrium mixture and placing said solid mass on said substrate; wicking off said peritectic liquid using said substrate, leaving behind crystals of said high temperature superconductor on said substrate; and removing said crystals from said substrate.
 2. The method as defined in claim 1 wherein said step of preparing precursor material comprises preparing a mixture of materials together which correspond to said selected composition.
 3. The method as defined in claim 2 wherein said high temperature superconductor comprise 123 Nd Ba Cu oxide and said precursor material consists essentially of Nd₂ O₃, CuO and BaCO₃ in the mole percent ratio of 1:10:4.
 4. The method as defined in claim 1 wherein said high temperature superconductor is selected from the group consisting of 123 NdBaCu oxide, 123 SmBaCu oxide and 123 YBaCu oxide.
 5. The method as defined in claim 1 wherein said step of heating said precursor material comprises heating to a peritectic phase region and forming and growing said crystals.
 6. The method as defined in claim 5 wherein said step of growing said crystals includes slowly cooling said mixture of peritectic liquid and crystals at least through said peritectic phase region.
 7. The method as defined in claim 1 wherein said crucible consists essentially of a material inert to said peritectic liquid and crystals.
 8. The method as defined in claim 1 wherein said substrate comprises a material inert to said peritectic liquid and crystals.
 9. The method as defined in claim 8 wherein said substrate comprises a higher melting point compound consisting essentially of elemental constituents present in said high temperature superconductor.
 10. A method of preparing high temperature superconductor single crystals, comprising the steps of:preparing a precursor material having a selected composition; heating said precursor material and achieving a peritectic mixture of phases including a peritectic liquid and crystals of high temperature superconductor; positioning near said peritectic mixture a substrate having a porosity and being inert to said peritectic mixture; quenching said peritectic mixture on said substrate; wicking off said peritectic liquid using said substrate, leaving behind crystals of said high temperature superconductor on said substrate; and removing said crystals from said substrate.
 11. The method as defined in claim 10 wherein said precursor material comprises a mixture of compounds.
 12. The method as defined in claim 10 wherein said high temperature superconductor is selected from the group consisting of 123 NdBaCu oxide, 123 SmBaCu oxide and 123 YBaCu oxide.
 13. The method as defined in claim 10 wherein said step of heating comprises heating to a peritectic phase region, forming said crystals and growing said crystals.
 14. The method as defined in claim 13 wherein said step of growing said crystals comprises slowly cooling said peritectic mixture.
 15. The method as defined in claim 10 wherein said step of heating includes placing said precursor material in an inert crucible.
 16. The method as defined in claim 10 wherein said inert crucible is selected from the group consisting of alumina, a noble metal and a compound having elements of said high temperature superconductor with a melting point higher than said temperature superconductor.
 17. The method as defined in claim 10 further including the step of using adjusted compositions for said precursor material for forming different morphologies for said crystals.
 18. The method as defined in claim 10 further including the step of adjusting crystal growing conditions for forming different morphologies for said crystals. 